A Contract of Affreightment creates a very different planning problem from an open spot market. Cargo liftings are not simply opportunities that may or may not be selected, because they represent transportation commitments that have to be covered within the agreed framework.
This changes the role of optimization. Instead of asking which cargo is the most attractive, the planner has to determine which vessel should perform each lifting, in what sequence, and how the complete program can be executed without creating unnecessary ballast, idle time or conflicts with future obligations.
Optimization Under Obligation
In a spot voyage problem, rejecting an unattractive cargo can be a perfectly valid solution. In a COA program, mandatory liftings remain part of the problem regardless of whether they are convenient for the current fleet position.
The flexibility therefore lies somewhere else. The optimizer can work with vessel assignment, sequence, timing and fleet deployment, while the underlying transportation obligations remain fixed.
Two Layers of the Same Planning Problem
COA planning can be viewed as the interaction of two different layers. The first is the obligation layer, consisting of mandatory liftings that have to be executed, while the second is the opportunity layer, where additional commercial employment may improve the use of the available fleet.
These layers cannot be optimized independently. A spot opportunity may look attractive by itself, but it only makes sense inside a COA plan if taking it does not compromise the vessel's ability to reach the next mandatory lifting in time.
Fleet Assignment Is a System-Level Decision
A COA program may involve several vessels and several liftings distributed across different dates and regions. Assigning each lifting independently can easily create a poor overall plan because a locally convenient vessel may be the only vessel capable of covering another obligation later.
Marine Solver evaluates vessel assignments as part of the complete transportation program. The question is not only whether a vessel can perform a particular lifting, but how that assignment changes the feasibility and efficiency of all subsequent liftings.
Sequence Matters as Much as Assignment
Two plans using the same fleet and covering the same liftings can produce very different results if the sequence changes. Each discharge position becomes the starting position for the next employment, while every additional day affects the vessel's ability to meet the next required time window.
For this reason, COA optimization is also a sequencing problem. Vessel assignment and lifting order have to be considered together because the quality of one decision depends directly on what follows it.
Timing Defines the Feasible Program
Contractual transportation programs are structured around time. Liftings may have defined windows, vessel availability changes throughout the planning horizon, and a sequence that works geographically can still become impossible if the vessel arrives too late for a later obligation.
Marine Solver includes timing constraints directly in the optimization model. Laycan logic, vessel availability, safety buffers, sailing time, port operations and waiting therefore influence which fleet schedules are actually feasible.
Vessel Position Is a Strategic Resource
The difficulty of a future lifting depends strongly on where the fleet is positioned before it begins. A technically suitable vessel may still be a poor candidate if reaching the loading region requires excessive ballast or makes another contractual obligation impossible afterwards.
COA planning therefore cannot separate the transportation obligation from vessel positioning. Every completed voyage changes the geographical state of the fleet and consequently changes the set of realistic decisions available for the next lifting.
Spot Cargoes Around Contractual Commitments
Mandatory liftings do not necessarily occupy the entire planning horizon. Depending on vessel positions and timing, gaps can appear before or between contractual commitments, creating room for additional spot employment.
Marine Solver can evaluate spot cargo opportunities within this existing COA structure. A spot cargo is considered as an optional addition to the plan, not as a replacement for a mandatory lifting, and it remains useful only if the contractual schedule stays feasible.
A Profitable Spot Cargo Can Still Be the Wrong Decision
This is one of the important differences between isolated voyage calculation and COA fleet optimization. A spot voyage can produce a positive result by itself and still weaken the complete transportation program if it moves the vessel into the wrong region or consumes time needed for a later lifting.
The value of the spot cargo therefore has to be measured inside the complete chain. Its contribution depends not only on its own freight and voyage cost, but also on what it does to the remaining obligations.
Fixed Obligations, Flexible Execution
COA contracts reduce commercial freedom in one sense because the liftings have to be performed. At the same time, the way the fleet executes those obligations can still contain a large number of alternatives involving different vessel assignments, sequences and intermediate employments.
This is where optimization becomes valuable. The objective is not to question the obligation itself, but to find a stronger execution structure within the freedom that remains.
Commercial and Operational Costs Stay Connected
A COA schedule cannot be evaluated only by counting completed liftings. Different fleet structures create different ballast legs, sailing times, waiting periods, port calls and fuel consumption, and these differences can materially change the economics of the complete program.
Marine Solver evaluates voyage and fleet costs within the same optimization framework. Fuel consumption, port expenses, canal costs, waiting and other voyage components remain connected to the assignment and sequence decisions that produced them.
Environmental Exposure Is Part of the Same Voyage Structure
Environmental and regulatory exposure also depends on the chosen fleet schedule. Different vessel assignments and voyage sequences change distances, fuel consumption, geographical exposure and the distribution of operations between regulatory regions.
Marine Solver can evaluate these consequences together with the commercial result, including EU ETS exposure, CII/AER assessment and FuelEU Maritime planning metrics. Environmental performance is therefore linked to the same physical voyage structure rather than calculated independently from the fleet plan.
Different Objectives Can Produce Different COA Plans
The strongest way to execute a transportation program depends on what the operator is trying to achieve. A cost-oriented schedule may distribute liftings differently from a plan focused on profitability, non-productive time or emissions.
Marine Solver can evaluate the same transportation obligations under different optimization objectives. The comparison can therefore reveal not only a numerical difference in the result, but a different structure of vessel assignments and voyage sequences.
Several Feasible Plans Can Be Commercially Relevant
A complex COA program does not always have one solution that dominates every alternative. Two fleet schedules may be close in cost while differing considerably in vessel positioning, timing reserves or the amount of flexibility left for future market opportunities.
For this reason, alternative feasible scenarios can be useful even when one mathematical optimum has already been identified. The planner can compare how different execution structures affect the complete transportation program before making commercial commitments.
Why the Problem Becomes Difficult So Quickly
Every additional lifting increases not only the number of possible vessel assignments but also the number of possible sequences. Adding more vessels and optional spot cargoes creates another layer of combinations because each choice changes the position and availability of the fleet for the decisions that follow.
The resulting problem is therefore not a collection of voyage calculations. It is a connected fleet optimization problem in which contractual obligations, timing, vessel positions and optional market opportunities influence one another.
From Reactive Execution to Program Planning
Without a fleet-level model, COA execution can easily become reactive. One lifting is assigned, the vessel completes it, and only then does the next constraint become fully visible, even though the consequences of the earlier decision were already embedded in the fleet position.
Optimization allows the program to be considered further ahead. Instead of solving each obligation only when it becomes urgent, the planner can evaluate how current assignments shape the feasibility and economics of the obligations that follow.
Pre-Fixture Decision Support
The role of Marine Solver is not to automate the commercial management of a COA contract. Its role is to make the consequences of different fleet deployment decisions visible before those decisions become fixed.
The model helps answer questions such as which vessel should perform each lifting, how the liftings should be sequenced, where optional spot employment can be inserted, and how those decisions affect the complete transportation program.
Conclusion
COA optimization is fundamentally a problem of executing obligations with limited and continuously moving resources. The liftings are fixed elements of the commercial program, but the fleet structure used to perform them remains a decision.
By combining mandatory liftings, vessel availability, sequence, positioning, voyage economics and optional spot opportunities within one model, Marine Solver turns COA execution into a structured fleet planning problem. The objective is not simply to complete every lifting, but to understand how the complete program can be executed more efficiently as a connected system.
Explore other Marine Solver planning models:
For full-vessel cargo selection and voyage-chain planning, read about the
Sole Cargo Module.
For multi-parcel transportation across Tanker, Bulk and General Cargo operations, explore the
Part Cargo Module.
For fleet-level transportation program coverage without fixed contractual liftings, see
Trader Fleet.